Well Drilling Method for Prevention of Lost Circulation of Drilling Muds

ABSTRACT

An improved well drilling method for directional drilling in subterranean formations which comprises incorporating into the drilling fluid an effective amount of fibrillated hydrophobic polypropylene fibers to form a suspension and injecting the suspension into the drill string whereby debris is swept from the borehole and seepage of drilling fluid into the subterranean formation is reduced.

BACKGROUND OF THE INVENTION

The present invention relates to a method for stopping the loss of drilling fluid circulating in well bores, and, more particularly, to the prevention of lost circulation due to all causes during directional, i.e., non-vertical, drilling in subterranean formations.

Most oil and gas reservoirs are much more extensive in their horizontal dimensions than in their vertical, i.e., thickness, dimension. By drilling a well which intersects such a reservoir parallel to its plane of more extensive dimension, horizontal drilling may expose more reservoir rock to the well bore than would be the case with a conventional well that penetrates the reservoir perpendicular to its plane of more extensive dimension.

The initial vertical portion of a directional, or horizontal, well is typically drilled using the same rotary drilling technique that has been used to drill most vertical wells, wherein the drill string is rotated at the surface. The drill string consists of many joints of steel alloy pipe, drill collars, and the drill bit itself. From the entry point of the vertical section into the curved section of the directional well, the curved section is drilled using a hydraulic motor mounted directly above the bit and powered by the drilling fluid. The current generation of horizontal drilling capability that can attain longer, deeper and more accurate placement of multiple horizontal well bores has also produced new problems in adapting conventional drilling fluids to overcome situations of lost circulation and/or debris removal in non-vertical bore holes. Lost circulation can occur as seepage losses, i.e., a slow, steady loss of drilling fluid into the zone during the process of drilling a well; or it can occur as a partial loss of fluid wherein drilling mud introduced into the borehole is, for whatever reason, simply not capable of being recovered; or the loss of drilling fluid can be an immediate, catastrophic loss that can result from a fracture in a given subterranean zone. The nature of horizontal drilling also produces new problems in that debris may gradually form into sediment along the bottom section of the elongated borehole which, in turn, tends to reduce drilling fluid circulation and requires more pressure to be exerted for the drilling fluid to continue flowing properly. Increased pressure, however, can also cause ruptures in the mud cake surrounding the borehole, which may also lead to significant drilling fluid losses due to subterranean zone fracture.

SUMMARY OF THE INVENTION

The present invention according to one embodiment is an improved well drilling method for drilling in subterranean formations wherein a drilling fluid is injected into a drill string having a bottom hole assembly inserted into a borehole and the borehole can deviate from a vertical orientation by as much as 90 degrees. The improvement comprises incorporating into the drilling fluid an effective amount of fibrillated hydrophobic polypropylene fibers and injecting the fluid and fiber combination, i.e., the suspension, into the borehole whereby debris is more efficiently swept from the borehole and seepage, i.e., lost circulation, of drilling fluid into the subterranean formation is simultaneously reduced and/or eliminated.

According to another embodiment, the present invention is an improved method for simultaneously sweeping debris from a borehole and reducing seepage and/or lost circulation into subterranean formations using a circulating drilling fluid or drilling mud. The invention is particularly applicable in a directional drilling process wherein the orientation of the borehole in the subterranean formation can deviate from vertical by as much as 90 degrees, i.e., the borehole curves from vertical and can become horizontal. The improved method comprises incorporating into the drilling fluid an effective amount of fibrillated hydrophobic polypropylene fibers to form a suspension, optionally with one or more other additives, and then circulating the suspension through the borehole. The optional one or more other additives may be incorporated directly into the suspension of fibrillated hydrophobic polypropylene fibers, or they may be circulated through the borehole in succession, depending on the particular drilling situation encountered.

An effective amount of fibrillated hydrophobic polypropylene fibers can vary over a wide range depending on many factors known to those skilled in the art of conventional and directional drilling, e.g., drilling fluid engineers, but typically the amount may range from 1.5 lbs (0.68 kg) up to 50 lbs (22.7 kg) of fibrillated hydrophobic polypropylene fibers per joint of pipe in the drill string. The invention is effective is controlling all situations of lost circulation, such as more gradual seepage losses, i.e., a slow, steady loss of drilling fluid into the zone during the process of drilling a well; or the loss of drilling fluid that may result from any other cause, such as an immediate, catastrophic loss event, e.g., an unexpected fracture in a given subterranean zone.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an sectional elevation view of a directional well in comparison to a vertical well.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an improved well drilling method particularly suited for directional drilling in subterranean formations wherein an effective amount of fibrillated hydrophobic polypropylene fibers is incorporated into the drilling fluid to form a suspension, and the fluid and fiber suspension is injected into the drill string having a bottom hole assembly inserted into the borehole. Debris is swept from the borehole and seepage of drilling fluid into the subterranean formation is reduced.

Drilling fluid, or “mud” is a vital part of drilling operations. Its composition, which can vary over a wide range during any drilling operation depending on peculiarities in the subterranean formation being drilled, is continually evaluated based on many factors. The composition needs to be selected to almost simultaneously provide (i) appropriate hydrostatic pressure on the borehole wall to prevent uncontrolled production of reservoir fluids, (ii) lubrication and cooling of the drill bit, (iii) carrying or “sweeping” of the drill cuttings from the bottom of the borehole up to the surface, and (iv) consistency for forming a “mud-cake” on the interior surface of the borehole. Sealing the interior surface of the borehole can be critical to prevent drilling fluid invasion of, or seepage into, the surrounding subterranean formation.

As drilling proceeds, the mud tends to accumulate small particles of the rocks which are being drilled through, and its properties can change. One of the most important mud properties is the mud weight, i.e., density. If the mud exceeds the fracture pressure of the formation being drilled, the formation may fracture, or fracture prematurely, and large quantities of mud can be lost into the formation. This is a situation that is more typically referred to as “lost circulation”, and lost circulation, which can occur unexpectedly without prior warning, has been observed to be particularly troublesome to correct in horizontal drilling operations.

As used in describing the present invention, however, the term “lost circulation” is intended to broadly cover all fluid loss situations that can occur, including seepage losses, i.e., a slow, steady loss of drilling fluid into the zone during the process of drilling a well; or a partial loss of fluid wherein drilling mud introduced into the borehole is, for whatever reason, simply not capable of being recovered; and the catastrophic loss of drilling fluid that can result from a fracture in a given zone.

Referring now to FIG. 1, the initial vertical portion 10 of a horizontal well A is typically drilled using the same rotary drilling technique that is used to drill most vertical wells B. The entire drill string is rotated at the surface. From a “kickoff” point 12 to an entry point 14 of the target oil reserve, the curved section 16 of a horizontal well is drilled using a hydraulic motor mounted directly above the bit and powered by the drilling fluid. The drill bit can be rotated by the hydraulic motor without rotating the drill pipe from the motor to the surface. Current horizontal drilling technique uses a steerable downhole motor (not shown). By orienting the bend in the motor, the hole can be steered around a curve, or bend, from vertical to horizontal, with the curved section typically having a radius of from 300-500 feet (91.4-152.4 meters). Somewhat unique to horizontal drilling is that debris may gradually form into sediment along the bottom section of the bend and along the bottom section of the elongated horizontal, or near horizontal, borehole which, in turn, tends to reduce the available annular volume for drilling fluid circulation. This reduction in volume translates to requiring more pressure to be exerted for the drilling fluid to continue flowing properly and sweeping debris from the borehole.

According to the present invention, conditions encountered in horizontal drilling can be quickly and satisfactorily resolved by incorporating into the circulating drilling mud from 1.5 lbs (0.68 kg) up to 50 lbs (22.7 kg) of fibrillated hydrophobic polypropylene fibers per joint of pipe in the drill string. The combination of a conventional drilling fluid with a precise amount of fibrillated hydrophobic polypropylene fibers is directed, i.e., forced, through sections of the well bore where a condition of lost circulation or sweeping difficulty has been detected. The fibrillated hydrophobic polypropylene fibers form a suspension in the drilling mud that is freely pumpable. The fibers have been observed to exhibit a structural stiffness that is capable of forming a fibrous mat in the region of lost circulation to thereby seal the void. The tendency for debris to form into sediment along the bend and elongated sections of a directional borehole has also been overcome using the fiber suspension of the invention, with or without optional additives, in place of a conventional drilling mud.

In a preferred embodiment of the invention, the suspension of fibrillated hydrophobic polypropylene fibers is first circulated through the well bore. The results are closely monitored, and if they are not completely satisfactory, the suspension of fibrillated hydrophobic polypropylene fibers is followed by one or more suspensions of other additives selected from the group consisting of cedar fibers, mica, cotton seed hulls and mixtures of such additives. The order in which the suspensions of other additives are circulated through the well bore is not critical, and will be selected depending on the analysis of subterranean formation being drilled.

As used herein, the term “fibrillated” is intended to mean that the fiber has been abraded and fibrils have been created along the fiber's length. The fibers contemplated for use according to the invention may also be referred to as fibrillated fibrous structures. A fiber tow is chopped to a specific length, usually in the range of about 1 millimeter to about 8 millimeters, although the length can vary over a wide range. The chopped fibers are fibrillated in a device having characteristics similar to a blender, or on a large scale, in machines commonly referred to as a “hi-low”, a “beater” or a “refiner”. The fiber is subjected to repetitive stresses, while further chopping and the reduction of fiber length is minimized. As the fibers undergo these stresses, the synthetic fibers tend to split as a result of weaknesses between amorphous and crystalline regions, thus becoming fibrillated. A preferred fibrillated fiber for use according to the invention is a 100 percent virgin homopolymer polypropylene fibrillated fiber available commercially as FIBERMESH® 300 Synthetic Fiber from Propex Concrete Systems. The polypropylene fibrillated fiber is hydrophobic and contains no reprocessed olefin materials.

The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. 

1. An improved well drilling method for drilling in subterranean formations wherein a drilling fluid is injected into a drill string having a bottom hole assembly inserted into a borehole and the borehole can deviate from a vertical orientation by as much as 90 degrees, the improvement which comprises incorporating into the drilling fluid an effective amount of fibrillated hydrophobic polypropylene fibers to form a suspension and circulating the suspension through the drill string whereby debris is swept from the borehole and loss of drilling fluid into the subterranean formation is reduced.
 2. The improvement of claim 1 wherein an effective amount of fibrillated hydrophobic polypropylene fibers is from 1.5 lbs up to 50 lbs of fibrillated hydrophobic polypropylene fibers per joint of pipe in the drill string.
 3. The improvement of claim 2 wherein the circulating suspension of fibrillated hydrophobic polypropylene fibers is immediately followed by circulating one or more suspensions of other additives selected from the group consisting of cedar fibers, mica, cotton seed hulls and mixtures of such additives.
 4. In a directional well drilling process wherein the orientation of the borehole can deviate from vertical by as much as 90 degrees, an improved method for simultaneously sweeping debris from the borehole with a circulating drilling fluid while reducing lost circulation which comprises incorporating into the drilling fluid an effective amount of fibrillated hydrophobic polypropylene fibers.
 5. The method of claim 4 wherein an effective amount of fibrillated hydrophobic polypropylene fibers is from 1.5 lbs up to 50 lbs of fibrillated hydrophobic polypropylene fibers per joint of pipe in the drill string.
 6. The method of claim 5 which includes the additional step of circulating one or more suspensions of other additives selected from the group consisting of cedar fibers, mica, cotton seed hulls and mixtures of such additives in series with said fibrillated hydrophobic polypropylene fibers. 